1. Field of the Invention
This invention relates to a free electron laser (FEL) and more specifically to a FEL in which short wavelength laser radiation is produced using low-energy recirculating DC electron beams and in which a continuous electromagnetic pump field replaces the conventional static periodic magnetic field.
2. Description of the Prior Art
In a conventional FEL using static periodic magnetic fields the operating wavelength in the weak-field limit is given approximately by EQU .lambda..apprxeq.(.lambda.q/2.gamma..sup.2) (1)
where
.lambda.q=the period of the magnetic structure. PA1 .gamma.=electron energy measured in units of electron rest mass.
Using low energy electron beams, E.ltoreq.5 MeV conventional FEL's are limited to operation at relatively long wavelengths since magnetic structures having spatial periodicity shorter than .lambda.q=1 cm are impractical to construct. Using high energy electron beams, conventional FEL's can operate at shorter wavelengths but radio frequency (RF) accelerators are necessary to produce the high electron energies.
The above limitations are avoided by the instant invention in which the proven technology of electrostatic accelerators is combined with low energy recirculating DC electron beams, to provide a continuous wave (CW) FEL having tunable high power laser radiation at short wavelength, e.g. 1000 .ANG.&lt;.lambda.&lt;50.mu.. This is achieved by replacing (2) the static periodic magnetic field used in conventional FEL's by an equivalent intense and continuous electromagnetic pump field having a spatial periodicity or wavelength, .lambda.p, less than 1 centimeter. With a real pump electromagnetic field, the FEL output wavelength and the pump field wavelength are related in the weak field limit by the equation: EQU .lambda..degree.(.lambda.p/4.gamma..sup.2) (3)
Thus, for the same electron energy, .gamma., the real pump field yields shorter wavelength laser operation than the one obtained using a periodic magnetic structure.
Using low energy electron beams to produce short wavelength laser radiation has several practical advantages. First, the electron beams can be easily produced using the well understood and reliable technology of conventional electrostatic generators. Second, the quality of the emergent electron beams so produced by the electrostatic generators is excellent and practically all the electrons in the beam can participate in the wave amplification process. Third, the output electron beam can be decelerated, captured and re-accelerated to provide a fresh high quality electron beam which can be used repeatedly to produce high power optical radiation. Fourth, only a relatively low voltage DC power supply is required to replenish the energy lost by the electron beam, mostly due to laser radiation, and hence a FEL system operating with good efficiency is achieved. Finally, it is possible to generate continuous electron beams with electrostatic accelerators, hence, true CW operation of the laser can be achieved, as compared to the pulsed structure inherent in electron and laser beams produced by RF accelerators.